1. Field of the Invention
The present invention relates to a spindle motor for rotating recording disks and to a disk clamp mechanism used in the spindle motor.
2. Description of the Related Art
Spindle motors comprise a bracket mounted on a frame of a driving apparatus, a hub which is rotational relative to the bracket, and bearing means disposed between the bracket and the hub. A rotor magnet is mounted on the inner surface of the hub, and a stator is disposed facing the rotor magnet. Recording members, such as magnetic disks, are mounted on the hub.
In the spindle motor of the prior art, the entire hub is formed of an aluminum or iron material, which is the cause of the following problems. When a hub is formed of, for example, an aluminum material, a magnetic material (e.g., yoke member) must be disposed between the rotor magnet and the hub in order to prevent magnetic fluxes from leaking from the magnetic circuit to the motor. It is therefore difficult to miniaturize the motor. When the hub member is formed of an iron material, machining it is difficult, particularly machining the surface on which recording disks are placed.
This type of spindle motor has the drawbacks described below. One of the ends of the rotor magnet is positioned in the upper section of the inner surface of the bracket. The height of the components including the rotor magnet and the bracket is slightly greater than the sum of the height of both the rotor magnet and the bracket (that is, greater by at least as much as the gap between them). Therefore, the over-all height of the motor becomes greater, thereby making it difficult to make the motor thinner. To make a motor thinner, such things as making the thickness of the bracket thinner, or making the height of the rotor magnet less, can be considered. However, when the bracket is made thinner, its strength is decreased; hence a new problem, such as vibrations, arises. When the height of the rotor magnet is reduced, a problem arises in that the torque of the motor is decreased. To prevent the vibration of the motor, the magnetic center of the stator has to be substantially matched with that of the rotor magnet. In the prior art, the magnetic center is adjusted by regulating the number of pieces of core plates. Therefore, it is difficult to accurately make the magnetic centers coincide with each other. Depending upon the construction of the motor, it is fairly difficult to make the magnetic centers of the stator and the rotor magnet coincide.
A clamp mechanism used in this type of spindle motor includes a disk placement surface provided on a hub, and a clamp member mounted on this hub. A flat placement surface is provided on the hub, and a flat pressing surface is provided in the clamp member in correspondence with the placement surface. However, it is difficult to machine the placement surface and the pressing surface flat and with a high degree of accuracy. Therefore, it is difficult to support recording disks in such a manner that the surfaces thereof are substantially perpendicular to the axis of the rotation center of the hub. In particular, when there is only one recording disk, it is likely to change its shape according to the shapes of the placement and pressing surfaces. A problem arises when the recording disks are clamped, they change their shape if the placement surface and/or the pressing surface is not fairly flat.